Rolling bearings typically include a spacer means, usually called a cage, separator or retainer, which serves to space the rolling elements from each other uniformly around the periphery of the races. For some bearings designed to operate at high values of DN, the cage is guided by having it operate in close proximity to land or shoulder regions on either the inner race or the outer race. There is a close radial clearance, on the order of 0.010 inch, between the cage and the land regions of the guiding race. This design keeps the cage more nearly coaxial with the bearing, which improves the uniformity of spacing of the rolling elements, and also reduces vibration or wobbling of the cage. The bearing races are typically made of steel. Various developments have been undertaken to the bearing races or the roller elements to increase the life of the roller bearings.
A well-known steel used for bearing applications in aircraft engine steels is M50 and its low carbon, high nickel variation M50NiL. The limits of this material have been identified in U.S. Pat. No. 5,560,787 to Takagi. The problem with M50 is that is has low toughness since the material is through hardened. M50NiL, while an improvement, has been identified as having insufficient hot hardness, although it does provide improved core toughness and surface compressive stress region.
Among the developments include the application of hard, wear-resistant coatings, such as, for example, titanium nitride as set forth in U.S. Pat. No. 5,165,804 to Fisher et al., assigned to the assignee of the present invention and incorporated herein by reference. Other additive coatings that have been utilized include coating layers of various wear resistant materials, for example, alternating layers of TiN and NbN. such as set forth in U.S. Pat. No. 5,593,234 to Liston. One of the problems with coatings, even thin, wear resistant coatings such as titanium nitride, such as applied to M50 bearings, is that there is a tendency for such coatings to spall off as the bearing wears. Of course, there is a tendency for the spalling to increase as the coating layer is increased. This spallation occurs over a shorter period of time as the loads are raised and the rotating speeds are increased.
Other developments have included new materials for use in roller bearings. These new materials have been identified as having superior properties over the widely applied M50 and M50NiL steels typically used in aircraft applications. One such steel is identified in U.S. Pat. No. 5,560,787 to Takagi et al., which is touted as superior in toughness and hardness as compared to the M50 steels. This steel also can be carburized or carbonitrided to extend its service life at 400° F. and at high speeds. Another specially developed steel is disclosed in U.S. Pat. No. 4,930,909 to Murakami et al. This medium carbon steel is carburized or carbonitrided to include a case of retained austenite.
Still other efforts have been made to case-harden widely used bearing steels such as M50 and M50NiL. One such effort is set forth in U.S. Pat. No. 5,352,303 to Murakami et al. which carbonitrides the surface of a roller bearing to achieve a ratio of carbon and nitrogen at the surface within well-defined parameters. More recently, case-hardening has included dispersing TiN and TiCN particles within the roller bearing to improve the abrasion resistance. The material is then carbonitrided or carburized at temperatures above 1800° F. In U.S. Pat. No. 6,171,411 B1 to Okita et al., an alloy steel is carburized or carbonitrided to produce a surface with good wear resistance. The basic problem with the carbonitriding treatments is that such treatments tend to oversaturate the surface of the material, which typically includes the critical bearing surfaces. This oversaturation undesirably produces intergranular nitrides. As bearings which include the formation of such intergranular nitrides wear, they tend to spall, sometimes quickly
What is desired is a method that increases the Rockwell hardness (HRC) of M50 and M50NiL steels above current hardness values, alters the microstructure, provides surface compressive stresses and minimizes intergranular nitrides to enable such steels to be effectively used as bearing materials for extended times at high temperatures, high loads and high speeds without being subjected to rapid spallation.